CAKEWALK sonar x3, Reference Manual

Page: 827 / 2094

827
Digital audio fundamentals
Editing audio
Recording a sound
To record digital audio, your computer monitors the electrical signal generated by a microphone (or
some other electroacoustical device). Because the signal is caused by a sound, the signal strength
varies in direct proportion to the sound’s waveform. The computer measures and saves the strength
of the electrical signal from the microphone, thus recording the waveform.
There are two important aspects of this measuring process. First is the
sampling rate , the rate at
which the computer saves measurements of the signal strength. It is a known fact of physics that
you must measure, or
sample , the signal at a rate at least twice that of the highest frequency you
want to capture. For example, suppose you want to record a moderately high note on a violin—say
the A whose fundamental frequency is 440 Hz and all overtones up to five times the fundamental.
The highest frequency you want to capture is 2,200 Hz, so you need to measure the electrical signal
from the microphone at least 4,400 times per second.
Since humans can hear frequencies well above 10 kHz, most sound cards and digital recording
systems are capable of sampling at much higher rates than that. Typical sampling rates used by
modern musicians and audio engineers are 22 kHz, 44.1 kHz, and 48 kHz. The 44.1 kHz rate is
called
CD-quality , since it is the rate used by audio compact discs.
The other important aspect of the measuring process is the sampling resolution . The sampling
resolution determines how accurately the amplitude of each sample is measured. At present, the
music industry has settled on a system that provides 65,536 different values to assign to the
amplitude of a waveform at any given instant. Thus, each sample saved by your computer requires 2
bytes (16 bits) to store, since it takes 2 bytes to store a number from –32,768 to 32,767. The scaling
of the electrical input signal level to amplitude value is determined by your audio hardware and by
the position of your input level control.
What if the amplitude of the sampled signal gets too high, such that a 16-bit number is not large
enough to represent it? What typically happens is that the signal is
clipped , cut off at the maximum
value.
Here is what a clipped waveform might look like:
Clipping is not usually desirable and may have unpleasant audible effects. Sudden irregularities in
the waveform of any type can cause clicks, pops, and distortion of the original sound.
See:
“Digital audio fundamentals” on page 822
“Recording audio” on page 293